It has been known that irregularities of an epitaxial layer that are created in an epitaxial growth process of a semiconductor silicon wafer are due to its scratches or its polishing induced defects (PIDs) that occur in a previous mirror-polishing process.
The scratches of a surface of the wafer after the mirror polishing are accompanied with dislocations, which are crystal defects. If epitaxial growth is performed on this wafer with the scratches, then the dislocations are propagated to the epitaxial layer to create dislocations also in the epitaxial layer, resulting in inferior quality of the epitaxial layer. It is therefore important that the wafer before the epitaxial growth after the mirror polishing has no scratch.
FIG. 7 shows captured epitaxial defects in an epitaxial layer that are generated in the epitaxial growth on a wafer (a polished wafer) after mirror polishing with damage such as a scratch. An enlarged view at the lower left shows damage lying at near the boundary between the epitaxial layer and a substrate.
When epitaxial growth is performed on PID, a projection part (a projection) influenced by the shape of the PID can be observed on the outermost surface of the epitaxial layer. It can be also observed that there is no defect such as a dislocation in the interior of the epitaxial layer right under this projection part, and the epitaxial layer has no disturbance of its crystallinity.
FIG. 8(A) shows an image of PID observed on a polished wafer with a laser microscope (MAGICS made by Lasertec Corporation). FIG. 8(B) shows an image of observation at the same coordinates as the PID after epitaxial growth is performed thereon. As shown in the image after the epitaxial growth, a projection due to the PID can be seen.
FIG. 9 shows the cross section of an epitaxial layer with a convex shape observed by a transmission electron microscope (TEM) at the same coordinates as PID after epitaxial growth is performed on a wafer having the PID. This result reveals that although the epitaxial layer has no defect, the outermost surface of the epitaxial layer is convex upward over a width of 200 nm and its height is about 2 to 3 nm.
In conventional techniques, defects such as scratches can be reduced if a mirror-polishing stock removal is sufficiently ensured.
PIDs, on the other hand, are usually reduced under conditions that a polishing apparatus and a polishing pad are well managed by various methods, for example, as disclosed in Patent Document 1. Exemplary techniques employed on the PIDs also include cleaning just before epitaxial growth after mirror polishing as disclosed in Patent Document 2.